(1) First we have to calculate the sludge age tRX needed, using: KS (1 +kdtRX)

and eliminating: KS + S


The critical sludge age is: KS + S0

(2) The reactor volume V follows from the substrate balance:

Umax S

Umax XS

(3) The concentration of excess sludge (without considering sludge thickening by sedimentation) follows from balances for S

and for X

o _ - QoX + UXV - kdXV After elimination of p: ^ v _ Qo(So-S)


104 475

(4) For kd_o and Xe_o (no bacteria in the overflow of the settler) the mass flow rate for the surplus sludge follows

During biodegradation of Qo(So-S) _ 475o kg h-1 MLSS a mass of 2o42-1647 _ 395 kg d-1 MLSS was removed by bacteria decay

Qixcx the critical removal rate of the excess sludge follows:

VX 7o9 ■ 2 Qex,cXr _ --_ —— _ 8197 kg d-1 MLSS

tRXC o.t/3

For a sludge removel of more than 82oo during some days all bacteria of the activated sludge plant were washed out.

148 | 6 Aerobic Wastewater Treatment in Activated Sludge Systems PROBLEM 6.2

The same municipal wastewater as described in Problem 6.1 must be treated in an activated sludge reactor by extended aeration. The amount of surplus sludge is to be reduced by a factor of 2, compared to the maximum value given by YX/S = 0.43 g MLSS (g COD)-1.

The sludge thickening ratio is nE = 2.5.

The recycle ratio is nR = 0.8.

1. Which sludge age must be selected?

2. Calculate the necessary reactor volume.

3. Compare these results with those of Problem 6.1.

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